SUMMARY A method for autotransplantation of the uterus of the guinea-pig to the abdominal wall is described. Complete uterine autotransplantation by this method caused extended oestrous cycles (mean 24·67 days). Similar transplantation but without disconnexion of the cervical end of the uterus gave rise to cycles of similar length (mean 24·75 days) but transplantation without disconnexion of the tubal ends allowed the occurrence of normal cycles (mean 16·20 days). The site of metabolism of the uterine luteolytic factor is discussed.